Print density control

ABSTRACT

Print hammers in a printer are energized from a source of electrical energy whose voltage may vary. The motor driving the type carrier is energized from an oscillator whose frequency is caused to vary with the voltage of the print hammer source to prevent misregistration between the print hammers and the type characters on the type carrier. The duration of the pulses energizing the print hammers is controlled by print scan pulses derived from timing marks on the type carrier to provide constant print density.

United States Patent Gilbert et a1.

[ Sept. 10, 1974 PRINT DENSITY CONTROL [73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: Mar. 26, 1973 [21] Appl. No.: 344,666

[52] US. Cl 101/93 C [51] B41j 5/22 [58] Field of Search 101/93 C, 111, 110;

[56] References Cited UNITED STATES PATENTS 3,172,353 3/1965 Helms 101/93 C 3,366,044 l/l968 Marsh 101/93 C 3,467,005 9/1969 Bernard 101/93 C PRINT LINE E BUFFER Q- t 3,513,774 5/1970 Pawletko et al. 101/93 C 3,573,589 4/1971 Berry 101/93 C X 3,575,107 4/1971 McDonell et al. 101/111 X 3,636,867 l/l972 Bonzano 101/93 C Primary Examiner--Robert E. Pulfrey Assistant Examiner-Edward M. Coven Attorney, Agent, or Firm-Francis V. Giolma [5 7] ABSTRACT Print hammers in a printer are energized from a source of electrical energy whose voltage may vary. The motor driving the type carrier is energized from an oscillator whose frequency is caused to vary with the voltage of the print hammer source to prevent misregistration between the print hammers and the type characters on the type carrier. The duration of the pulses energizing the print hammers is controlled by print scan pulses derived from timing marks on the type carrier to provide constant print density.

6 Claims, 4 Drawing Figures l l I E km 7 a a a if l HAMMER HAMMER 1 SELECT I LATCHES l l k I I CIRCUIT l h 12 s SUBCYCLE GATING RING ,54 2 185 RESET 40 186 (3STAGE) 5 i PULSE common.

PRINT DENSITY CONTROL BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates generally to printers and it relates in particular to printers having continuously moving type carrying elements and print hammers which are operated by electromagnets from a source whose voltage may vary.

2. Description of the Prior Art US. Pat. No. 3,183,830 which issued on May 18, 1965 to D. M. Fisher et al. discloses the use of delay devices to compensate for differences in flight times of different print hammers.

U.S. Pat. No. 3,513,774 which issued on May 26, 1970 to J. P. Pawletko et al. discloses modifying emitter pulses used to fire print hammers so as to compensate for source voltage variations.

Application Ser. No. 338,133 of Richard L. Gilbert et al. filed March 5, 1973 discloses varying the speed of the type carrier motor in accordance with variations in source voltage so as to prevent misregistration between type characters and print hammers which would otherwise be caused by variations of the source voltage energizing the print hammers.

Application Ser. No. 318,309 of Richard L. Gilbert et al. filed Dec. 26, 1972 discloses varying the pulse width of print hammer firing pulses in accordance with the forms thickness and the source voltage so as to maintain constant impact force.

SUMMARY OF THE INVENTION Generally stated it is an object of the invention to provide a printer having improved print quality.

More specifically it is an object of the invention to utilize a change in speed of the type carrier in a printer, which is caused to prevent misregistration between the type and the print hammers when the voltage of the source energizing the print hammers varies, for varying the duration of the firing pulse of the print hammer to maintain a uniform print density.

Another object of the invention is to utilize print scan pulses derived from an emitter of a type carrier which is driven at varying speeds with source voltage variations for controlling the duration of the pulses firing the print hammers of a printer.

It is also an object of this invention to purposely vary the speed of the type carrier in a printer having voltage sensitive print hammer electromagnets and for using the varying duration of print scan pulses derived from an emitter connected in driven relation with the type carrier for controlling the energy imparted to the print hammer.

Yet another object of the invention is to provide for using subscan pulses which are used to scan predetermined sub groups of print positions in sequence and impression control pulses derived from the same clock pulses as the subscan pulses to provide print hammer firing pulses which are a rational multiple of the subscan pulses.

The foregoing and other objects, features and advan- DESCRIPTION OF THE DRAWING In the drawing FIG. 1 is a schematic block diagram of a printer control system embodying the invention in one of its forms.

FIG. 2 is a schematic circuit diagram of logic circuitry which may be used in the pulse control circuit of FIG. 1.

FIG. 3 is a schematic block circuit diagram of the motor control circuit of FIG, 1 and FIG. 4 shows timing curves illustrating the timing relations of the circuitry in FIGS. 1 and 2 for different motor speeds.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 1 the reference numeral 10 denotes generally a high-speed printer apparatus of the type generally described in Pat. No. 2,993,437 which issued to F. M. Demer et al. on July 25, 1961 and has been modified for use with the present invention. As shown a type cartridge 11 comprising an endless band 12 having a plurality of type characters 13 thereon is mounted between spaced apart wheels 14 and 15 for movement past a plurality of print positions along a'print line on a document or form 22 upon which a printing operation is to be performed. A plurality of print hammers 20 is positioned one in each position along the print line for impacting the document 22 and a ribbon 24 against selected type characters 13 as they pass the different print positions. Timing marks 16 are provided on the band 12 which are scanned by a transducer 18 and used for producing timing signals for use in the control system.

Information to be printed is stored in a print line buffer 26 which is scanned by the usual X drivers 28 and Y drivers 30 under the control of a X ring 32 and a Y ring 33 as described inthe Demer, et al patent. A subcycle gating ring 34 is provided so that the print positions are scanned in a predetermined spaced order as described in the Demer, et al patent. By using a threestage subcycle gating ring the stages 1, 2 and 3 scanning the buffer 26 in that order as described in the Demer et al. patent stages 2, 3 and 1 of the subcycle gating ring may be used in that order with the stages being shifted one position to gate a pulse control system 40 for controlling the actual firing of selected ones of the print hammers 20 simultaneously for each subcycle, through hammer drivers 21 from a source of electrical energy +V, on the following subcycle. Instead of utilizing the hammer select circuit of the Demer et al. patent for directly firing the print hammers 20, the hammer select circuit 175 is used to set select ones of a plurality of latches 36 which are connected through AND circuits 38 to the different print hammers 20. The AND circuits 38 are gated in subcycle groups by the pulse control circuit 40 under the control of the subcycle gating ring 34 for actually firing the print hammers 20 g which are selected during the previous subcycle. The

to be operated when the cartridge is moved closer to or away from the type hammers 20.

The type belt or band 12 is operated by means of a motor 44 which is connected in driving relation with the pulley 14 and is energized through a motor control circuit 46 also connected to the source +V through conductors 28. The motor 44 may comprise a stepping motor which is operated by means of pulses applied to the operating windings thereof. The stepping motor 44 may be of the type described in the article entitled Characteristics of a Synchronous Conductor Motor by Arthur E. Snowdon and Elmer W. Madsen published in Applications and Industry, March, 1962, being the subject of paper 61-650 recommended by the AIEE Industrial Control Committee and approved by the AIEE Technical Operations Department for presentation at the AIEE Northeastern District Meeting, Hartford, Conn., May 17-19, 1961. Such a motor has a speed proportional to the rate at which pulses are applied to it and is normally substantially unaffected by variations in the value of the source voltage.

Referring to FIG. 3 is will be seen that the motor control circuit 46 comprises a pair of triggers 48A and 483 connected to operate as a binary counter in response to input pulses from an oscillator 50. The oscillator 50 is of a voltage sensitive type so that the frequency of the oscillator varies with the voltage of the energizing source. The outputs of the triggers 42A and 42B are connected to AND circuits 52A, 52A, 52B and 523 which are connected to the terminals A, A, B and B of the motor 44 respectively. Accordingly when the frequency of the oscillator 50 varies with voltage variations, the speed of operation of the triggers 48A and 48B will vary accordingly as will also the speed of the motor 44. This means that when the voltage of the source energizing the print hammers varies and the speed of operation of the print hammers increases or decreases accordingly, the speed of the motor is likewise changed accordingly so that the print hammers 20 operate in registration with the type characters 13 on the type belt 12. Details of this aspect of the invention are described in detail in the co-pending application, Ser. No. 338,133 of Richard L. Gilbert et al. filed March 5, 1973.

Referring to FIG. 2, a logic circuit is shown for use in the pulse control circuit 40 of FIG. 1. One such logic control circuit is used for each of the three subscans so that the description hereinafter will be limited to one such circuit. A pair of triggers 56 and 58 are shown connected generally in a cascade relation with the trigger 56 being connected for example to stage 2 f the subcycle gating ring 34 at the D input. The same input signal is applied through an inverter 60 to the set terminal S of the trigger 58, and impression control single shot 62, the timing of which is controlled by the potentiometer 42 is connected to the transducer 18 through conductor 185 and the output thereof is applied as clock pulses to the CL terminals of the triggers 56 and 58, the signal being applied through an inverter 64 to the trigger 58. The reset terminals R of the two triggers may be connected to a source of DC reset and the Q output of the trigger 58 being used to fire the print hammers of its subscan group.

The triggers 56 and 58 may be of the general type known as D-type edge-triggered flip-flops. The triggers are set by a negative going signal applied to the S set terminal and are reset by the application of-a negative going signal to the R or resetterminal. The state of the D input existing immediately prior to the rising of a clock pulse applied to the CL or clock terminal is transferred to the Q output when the clock signal rises. The configuration of D flip-flops or triggers shown in FIG. 2 will produce an output pulse duration at the Q terminal of flip-flop 58 which is a function of the subscan time applied to the D input and the duration of an impression control single shot pulse applied to the CL clock input from the impression control single shot 62. In particular, the hammer fire pulse duration T, is equal to 1.5 times the subscan period T,, plus the single shot time T As shown in FIG. 4 a subscan time (n) is produced which is equal in duration to the period of the subscan pulses T from the printer emitter l8. Subscan n time is the D input to flip-flop 56. The inversion of subscan n is the set input to flip-flop 58. The impression control single shot 62 is fired at the center of each subscan period (/2 p), being fired on the negative going edge of each subscan pulse.

Referring to FIG. 4, at the time t subscan n rises and flip-flop 58 is held in the set state from the time t t At t, time the rising edge of the pulse from the impression control single shot 62 sets the output Q1 of flipflop 56 to an up level since the D input of flip-flop 56 is up at the time t,. At t, time the output Q, drops since the input to D is down when the impression control single shot 62 rises at time t.,. When the clock input from the impression control single shot 62 to flip-flop 58 rises at t;, time the output Q (fire n) remains up since the D input to flip-flop 58 from Q is up during this transition. Fire n output from flip-flop 58 will fall at the next positive transition of the clock input from the impression control single shot 62 to flip-flop 58 or at t; time when the impression control single shot 62 times out and inverter 64 applies a positive signal to the CL terminal of flip-flop 58. T T l/2 T T, 1.5 T 7",. Hence if T varies (with belt velocity) T, will vary accordingly.

In operation the print line buffer 26 of FIG. 1 is scanned by the X drivers and the Y drivers under the control of the subcycle gating ring 34 to provide three separate subscans of all print positions with print positions 1, 4, 7, 10, etc. being scanned in the first subscan, and print positions 2, 5, 8, 11, etc. being scanned in the second subscan, while print positions 3, 6, 9, and 12, etc. are scanned in the third or last subscan. The hammer select circuit sets the different hammer latches 36 in response to a compare between a character contained in the print line buffer 26 for a given print position and a character on the type chain 12 in that position. The pulse control circuit 40 operates to gate the output of the selected latches 36 through the ANDs 38 to fire the selected print hammers 20. The pulse generation of the gating pulses applied to the ANDs 38 by the pulse control circuit 40 will be a function of the length of the individual print subscans and the duration of the impression control single shot pulse.

Referring to FIG. 4 curves a-i illustrate respectively, d-c reset, subscan pulse from the transducer 18, impression control single shot pulse from single shot 62, subscan n or 2 stage pulse from the subscan ring 34, the subscan n+1 pulse or 3 stage pulse from the ring 34, the inverted subscan n pulse, from inverter 60, the inverted single shot pulse from inverter 64, the fire n output from flip-flop 58 and the Q, output of flip-flop 56 with the motor 44 driving the type belt 13 at normal speed. When the voltage of the source +V from which the print hammer drivers 21 are energized drops, the time required for the print hammer to impact the document 22 is increased. The frequency of the oscillator 50 is reduced and the triggers 48A and 48B drive the stepping motor at a slower rate.

Accordingly the subscan pulses from transducer 18 are stretched out as shown at b and the pulses C from the impression control single shot 62 commence later (their length is a function of the setting of the potentiometer 42 in accordance with the thickness of the form or document). The subscan n pulse d, from ring 34 is stretched, and accordingly the duration of the fire n pulse h is increased so that the product of the reduced voltage applied to the print hammers and the pulse width remains substantially constant, thereby regulating the print hammer force and the print intensity.

The pulse control circuit 40 will supply the printer with the correct hammer fire pulse width which is derived from the two basic signals provided by the printed electronics. The first of these signals is the print subscan pulse which is derived from the timing marks 16 on the print element 12. The period of these pulses is proportional to the velocity of the print element 12.

The second signal is the output of the impression con- 2 trol single shot 62. This single shot is fired during every subscan time and its pulse width is porportional to the forms thickness. The using system must process these signals to develop separate but equal firing times T, with pulse width equal to the impression control single shot time T, 1 /2 subscans T,,. The single shot duration is fixed for a given form thickness but the subscan period varies with print element velocity. This results in a fire pulse width duration consistent with maintaining print density.

In a typical application the following timing has been found to produce satisfactory results Nominal subscan time T, is 710 microseconds.

Therefore, 1.5 T, equals 1,065 microseconds. Nominal hammer fire pulse width equals 1,200 microseconds for one and two part, 1,300 microseconds for three part and 1,450 microseconds for 4 and 6 part.

Therefore the portion of hammer fire pulse width which varies dependent upon voltage is between 74 and 89% of the total fire time.

To maintain constant flight time the velocity of the print element is varied plus or minus 5% for a power supply variation of plus or minus 10%. Therefore, the variation of pulse duration to maintain constant print force is between plus or minus 4.4% for one part and plus or minus 4.0% for six part forms. This has been successfully demonstrated to maintain nearly constant print density over the allowable range of power supply voltage variation.

From the above description and'the accompanying drawing it will be apparent that a new and novel printer control circuit is provided utilizing a variation in the speed of the driving motor for the type cartridge with changes of voltage applied to the print hammers to maintain registration between the type characters and the print hammers. The variable length of subscan pulses produced by the variation in speed of the type carrier is utilized to vary the duration of the pulses applied to the print hammers so that the product of voltage and time tends to remain constant thus maintaining a substantially constant hammer print force to minimize print density variations.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by-those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.

What is claimed is:

1. In a printer having a plurality of print hammers with firing means disposed to be energized from a source of electrical energy and a type carrier with a plurality of type characters movable past said print hammers, means for storing data to be printed in each of a plurality of print positions corresponding to said print hammers, means'for determining the character at a particular hammer position, and means for addressing said storage means and comparing the data in storage for the particular hammer position with the character on the carrier at said particular position and generating a print compare signal when they are the same to effect operation of a selected one of said print hammers, the improvement in hammer timing logic comprising:

means including a variable speed motor driving said type carrier to move said type characters past said print hammers,

motor control means connected to run said motor at a speed dependent on the voltage of said source,

and

pulse means including a transducer connected in operating relation with said type carrier to produce subscan pulses which vary in duration in response to changes in the speed of said type carrier and connected to address said storage means and effect energization of said print hammers in response to said print compare signal said variable duration subscan pulses thus controlling the duration of energization of said print hammer firing means in response to variations in the voltage of saidsource.

2. The invention as defined in claim 1 characterized by said motor control means comprising a voltage responsive oscillator.

3. The invention as defined in claim 2 characterized by said pulse means including an impression control single shot connected to said transducer and controlled by pulses from said transducer with said impression control single shot being connected so that its output pulse is cumulative with said subscan pulses.

4. The invention as defined in claim 3 characterized by said pulse means including a multi-stage counter producing said subscan pulses and bistable means connected to both said subscan counter and said print hammers to provide a print hammer firing pulse which is a rational multiple of said subscan pulses.

5. The invention as defined in claim 4 characterized by a thickness control circuit connected to said impression control single shot whereby said single shot has a duration controlled in accordance with the thickness of the document being printed on.

6. The invention as defined in claim 5 characterized by said bistable means comprising a pair of flip-flops connected in cascade to said multi-stage counter and said impression control single shot so as to be selectively controlled by said subscan pulses and said impression control pulses to provide a print hammer energizing pulse which'is a rational multiple of said subscan pulses and said impression control single shot having a controlled duration additive with respect to said rational multiple. 

1. In a printer having a plurality of print hammers with firing means disposed to be energized from a source of electrical energy and a type carrier with a plurality of type characters movable past said print hammers, means for storing data to be printed in each of a plurality of print positions corresponding to said print hammers, means for determining the character at a particular hammer position, and means for addressing said storage means and comparing the data in storage for the particular hammer position with the character on the carrier at said particular position and generating a print compare signal when they are the same to effect operation of a selected one of said print hammers, the improvement in hammer timing logic comprising: means including a variable speed motor driving said type carrier to move said type characters past said print hammers, motor contrOl means connected to run said motor at a speed dependent on the voltage of said source, and pulse means including a transducer connected in operating relation with said type carrier to produce subscan pulses which vary in duration in response to changes in the speed of said type carrier and connected to address said storage means and effect energization of said print hammers in response to said print compare signal said variable duration subscan pulses thus controlling the duration of energization of said print hammer firing means in response to variations in the voltage of said source.
 2. The invention as defined in claim 1 characterized by said motor control means comprising a voltage responsive oscillator.
 3. The invention as defined in claim 2 characterized by said pulse means including an impression control single shot connected to said transducer and controlled by pulses from said transducer with said impression control single shot being connected so that its output pulse is cumulative with said subscan pulses.
 4. The invention as defined in claim 3 characterized by said pulse means including a multi-stage counter producing said subscan pulses and bistable means connected to both said subscan counter and said print hammers to provide a print hammer firing pulse which is a rational multiple of said subscan pulses.
 5. The invention as defined in claim 4 characterized by a thickness control circuit connected to said impression control single shot whereby said single shot has a duration controlled in accordance with the thickness of the document being printed on.
 6. The invention as defined in claim 5 characterized by said bistable means comprising a pair of flip-flops connected in cascade to said multi-stage counter and said impression control single shot so as to be selectively controlled by said subscan pulses and said impression control pulses to provide a print hammer energizing pulse which is a rational multiple of said subscan pulses and said impression control single shot having a controlled duration additive with respect to said rational multiple. 